God's Architect: Pugin and the Building of Romantic Britain by Rosemary Hill 602pp, Allen Lane, £30

Augustus Welby Northmore Pugin didn't waste time. Born in 1812, the only child of a French artist and an intellectual English mother, he was encouraged from the start to look and to record. He began his study of medieval buildings at the age of six. At nine he was travelling in France with the pupils at his father's drawing school, sketching and measuring. At 12 he started collecting high-class antiquities, of the kind shaken free in large quantities by the French revolution, and at 14 was dealing in them. At 15, under his father's aegis, he designed a gigantic sideboard, vaguely resembling a Perpendicular Gothic rood screen, for George IV at Windsor Castle, as well as dining chairs so massive it took two footmen to move them; two years later he opened his own woodcarving business. At 19, he bought a boat, wrote his autobiography and got married. At 20, he became a father and, a week later, a widower.

He cannot have known that he would die, at 40 and insane - Rosemary Hill thinks it almost certain he had syphilis, contracted in his rackety teens. But his whole life has a manic dynamism that overwhelms the reader of this outstanding biography as it did those whom he came in contact with. He had little formal education and no architectural training: the great apologist for the revival of Gothic architecture was engagingly illiterate, his theories as much a matter of conviction as of learning. As Hill says of Contrasts, the satirical broadside that made his name in 1836: "He knew almost as much as anyone alive about medieval art and architecture, but he had never heard of the Renaissance" - indeed not hearing of the Renaissance would always be his line. His books, which fired up a whole generation of architects, were written in wild bursts, and revised only by the impatient addition of footnotes. He was a ferocious worker, putting in 16 hours a day, with an hour off for exercise, even during his debilitating later illnesses - Charles Barry marvelled at his "50-horsepower of creation". And he travelled incessantly, between jobs in England and Ireland, and to France, Germany and Belgium looking at medieval buildings. Of Louisa, his second wife and mother of five of his eight children, Hill remarks that he came home just often enough to keep her continually pregnant.

Pugin was an early convert to the English Catholic church, and his principal work was for Catholic patrons, even if his influence was inevitably to be felt most widely in the immense Anglican church-building boom of the Victorian era. In Contrasts, he juxtaposed an ideal of medieval life in a Catholic feudal England with that in the godless modern industrial city. But the book that electrified contemporaries such as Scott and Butterfield was The True Principles of Pointed or Christian Architecture of 1841, in which he expounded the far-reaching rules of honesty of construction and subordination of ornament to function. It was an assault on the merely picturesque use of Gothic and on much contemporary practice, and confirmed Pugin as a prophetic outsider. This seems to have been his preferred position, "internally convicted", as he put it, and possessed of what Hill wonderfully calls "overweening self-effacement". "There is no likelyhood of my ever having any letters after my name," he said, "unless VP (very pointed)." Pugin articulated the principled revolt against the plaster and putty world of the Regency in which he had grown up, and in which his own father had painted "perspectives", or artist's impressions, for John Nash, the great architectural showman of the age. It was a generational reaction as passionate and blinkered as that which would turn against Victorian taste 80 years later.

Pugin's own achievement as an architect is harder to assess. He has not left a corpus of indisputably great buildings. He himself was disappointed with almost everything he built, and the substantial work-list at the back of this biography is also a catalogue of later mutilations and demolitions. Hill emphasises the widespread types that he originated, the Victorian village school, church and rectory, but types are by their nature impersonal. Everyone knows he collaborated with Charles Barry on the Houses of Parliament, but his precise contribution is confounded with the other architect's, and was to be the subject of unseemly disputes between their sons after both were dead. As Hill says, they were "two perfectionists, with different ideas of perfection", and the brilliant designs for the interior that streamed from Pugin's pen were incessantly revised by Barry, and again by Pugin, and so on. But Pugin's official title was merely superintendent of woodcarving, and in the huge fanfare that attended the opening of the House of Lords in 1847, and that of the House of Commons five years later, his name went entirely unmentioned. It is clear that he was responsible for the Big Ben clock-tower, a design produced in his feverish last year, just before he lost his mind, but even after reading Hill's biography you are left unsure just how much the exterior design of the palace owes to him. The gilded vanes, which add so much to its fairytale skyline, are his, but if Barry, as Hill says, "could not design a door knob in the medieval style", how did he generate the profuse late Gothic detail of the rest?

To see Pugin uncompromised and unaltered you need to go to his church of St Giles at Cheadle in Staffordshire. It was his attempt to recreate "a complete English parish church of the time of Edward I", a project in which the archaeological takes on the air of a dream. Built low down in the small hillside town, it none the less dominates it with a tower and spire of subtly alarming slenderness and height. It was paid for by the Earl of Shrewsbury, of nearby Alton Towers, the most prominent Catholic layman in England and Pugin's most tireless benefactor and supporter. The interior of the church staggers you with the lavishness of its decoration, every surface painted, gilded or stencilled in rich colours. There is a rood screen, axiomatic to Pugin's view of the revived Catholic church; beyond it the chancel is a climax of gold, with canopied sedilia and an Easter sepulchre around which long-forgotten pre-reformation rites were to be revived. It is the triumph of the whole thing, in its spectacular exuberance, to appeal as much to feelings as to principles. The next stage on from such a church, exemplified by Butterfield's All Saints, Margaret Street, begun three years later, would be the adoption of the "constructional polychrome" advocated by Ruskin, of decor in brick and tile integral to the structure, not merely applied to it as if from an endless supply of wallpaper samples. The contrast between the two churches is telling, the excitement of All Saints sharpened by something crude and rebarbative that is quite absent in Pugin's childlike surfeit of delight.

In Pugin's later years, when architectural work fell off, he continued his revolutionary work in design, and the Medieval Court at the Great Exhibition of 1851 was a showcase of his long collaborations with such figures as the manufacturer John Hardman, the decorator John Crace, and Herbert Minton, whose encaustic tiles are so integral to our sense of Victorianism. As at Cheadle, every aspect of design deserved attention. "We must have a turn at CARPETS next," Pugin wrote to Crace; "let us reform them altogether." As so often, Pugin's ideas were fertile in application for generations yet unborn, for the Arts and Crafts movement and even for modernism.

Hill's absorbing book is a marvellously clear guide through the agitated density of Pugin's life and through the volatile worlds of early Victorian taste, politics and theology. The plates are excellent, though now and then it would be good to be told more fully and exactly what a building looked like; there is perhaps a degree of uncertainty as to how much the reader is expected to know about the subject already. But of Pugin himself, rough, emotional, magnetic, the picture is unforgettable. You watch him with enthralled anxiety as he tears through his short life, always pushing himself to the very limit in unselfconscious obedience to his own genius. Hill's account of his end is intensely moving, its stunned quietness a somehow inevitable culmination to a career of such prodigious energy.